This invention relates to the field of data processing systems. More particularly, this invention relates to data processing systems including a translation lookaside buffer mechanism having provision for the lockdown of entries within that translation lookaside buffer mechanism.
Translation lookaside buffers serve to map virtual addresses to physical addresses, as well as other functions, within a data processing system. One type of translation lookaside buffer is fully associative and uses a content addressable memory to store TAG values for each entry such that an input TAG value may be compared in parallel with all of the entries within the content addressable memory to find any match. If a match occurs, then the corresponding data entry, such as a virtual address, or a portion of a virtual address, may be read out from the data memory associated with the content addressable memory. Another known type of translation lookaside buffer is a set associative translation lookaside buffer in which a portion of the data value being looked up is used to select amongst a plurality of TAG RAMs with a different portion of the data value being used to index a particular entry within that TAG RAM before a comparison is made with the corresponding TAG data stored in the identified entry to check for a match. If such a match occurs, then a data value within a data RAM associated with the selected TAG RAM may be read out.
Lockdown mechanisms are provided for entries within translation lookaside buffers. Lockdown may be used such that once an entry has been placed into the translation lookaside buffer it will not be removed during normal processing operation and accordingly a guaranteed fast lookup of the translation required may be assured. Lockdown entries are particularly useful for handling translations associated with performance critical operations such as interrupt handling, abort or exception handlers and frequently used performance critical program code or data.
Fully associative translation lookaside buffers have the advantage that they provide a flexible environment in which entry lockdown can be supported in that a given new entry can be stored at any position within the content addressable memory and accordingly if a portion of the content addressable memory is locked down, then this does not unduly restrict the options for storing non-lockdown entries. However, a problem with content addressable memories is that in the increasingly common situation of synthesised designs, content addressable memories tend to have a disadvantageously large circuit area and power consumption requirement as well as being relatively slow.
Set associative translation lookaside buffers have the advantage that they are well suited for synthesised design since the circuit elements, largely RAM arrays, from which they are formed are readily provided within the cell libraries of circuit synthesis systems. Synthesised set associative translation lookaside buffers tend to be relatively small, power efficient and fast compared to large synthesised content addressable memories. However, a problem associated with set associative translation lookaside buffers is that a given entry has a finite, and often relatively small, number of positions within which it may potentially be stored within the set associative translation lookaside buffer. If one or more of these potential locations is locked down, then this can severely restrict the possibilities for storing a desired entry within the translation lookaside buffer and can significantly degrade performance. It is possible that in some situations all the potential storage locations for a new entry might already be occupied by locked down entries and accordingly storage of that entry within the translation lookaside buffer would not be possible at all. It is also possible that all of the alternative storage locations for an entry which it is desired to lock down within the set associative translation lookaside buffer might already be occupied by locked down entries and accordingly a highly performance critical entry may not be accommodated within the set associative translation lookaside buffer.